snapshot takes kwargs + better anim for PSV

snapshot takes kwargs and passes them along to plot_snapshot.
Added an explore method that creates an IPython widget to view time
slices. Also supports the kwargs and passes them along.

ElasticPSV.snapshot can take plottype arg with list of plot types that
will be overlaid. explore and animate take the same args and behave like
snapshot. This allows one to make animations with, e.g., the wavefield
overlaid by the displacement vectors.